Allylboranes achiral

The allylboranes used in Figure B5.6 are achiral and so only the relative stereochemistry of the products is controlled when they react with aldehydes. If homochiral allylboranes are used, however, then not only the relative... 

Table 10-13. Reaction of achiral aldehydes with chiral allylborane reagents [125e].

Corey and coworkers have described Ae preparation of allylborane (289) by the reaction of bromobo-rane (2W) and allyltributylstannane and shown that (289) undergoes highly stereoselective reactions with )x>th achiral (95-97% ee) and chiral aldehydes (Scheme 54). The corresponding methallyl, ( )-crotyl and 2-chloro- and 2-bromo-allyl reagents were prepared by similar methods and shown to give excellent results in reactions with achiral aldehydes (84-99% ee in most cases). 

Of all the chiral auxiliaries that have been studied so far, a-pinene-based allylboranes have proven to be one of the most effective, inexpensive, and versatile reagents based on high optical purity observed for the product homoallylic alcohols. " a-Pinene is highly inexpensive and readily obtained from pine trees. Both enantiomers of a-pinene are available in nature (-)-a-pinene is more common in Europe, and the (+)-a-isomer is found in North American pine trees. Moreover, a-pinene-based allylboranes were found to exhibit excellent levels of reagent-controlled stereoselectivity in their reaction with a wide variety of chiral and achiral aldehydes. These factors greatly simplify the synthetic design of complex natural products as the requisite chiral centers can be... 

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